This work seeks to clarify the biochemical mode of action of Se as an essential trace element and as a factor modifying the toxicity of environmental toxicants such as heavy metals. Ovine erythrocyte glutathione peroxidase is to be characterized with regard to the chemical structure and oxidation states of Se in the enzyme. The forms of Se in foods such as marine fish will be studied. The Se status of persons having cystic fibrosis will be investigated.